1. Field of the Invention
The present invention relates to a square pipe, a frame structure, a square pipe manufacturing method, and a square pipe manufacturing apparatus.
2. Description of the Related Art
There has heretofore been known a method for forming a square pipe having a closed rectangular cross section by the use of, for example, a metal plate such as a galvanized steel plate as a material (see, for example, Japanese Patent No. 3974324).
This forming method disclosed in Japanese Patent No. 3974324 is a method for forming a square pipe including a plurality of wall surfaces and having a rectangular cross section by use of an intermediate formed product. The intermediate formed product includes a bottom wall surface, side wall surfaces obtained by bending both sides of the bottom wall surface in its width direction at obtuse angles, and flange parts obtained by bending tip portions of the side wall surfaces. Specifically, the intermediate formed product is formed by bending a rectangular metal plate along a plurality of bend lines extending in a length direction of the metal plate. Particularly, a main feature of the method is to generate residual stress on the bottom wall surface of the intermediate formed product by curving the bottom wall surface into a downward convex shape and then planarizing the curved bottom wall surface by vertical compression. Here, the residual stress causes the flange parts forming a top wall surface to come into close contact with each other.
However, in the above forming method described in Japanese Patent No. 3974324, the residual stress generated on the bottom wall surface may cause the bottom wall surface of the completed square pipe to be curved into a convex or concave shape even though only slightly. Therefore, it has been difficult to obtain the rectangular cross section in a reliable manner. Moreover, there has been a problem that it is difficult for the residual stress on the bottom wall surface to bring the flange parts into close contact with each other surely and evenly.